Positron Emission Tomography (PET) relies upon the use of positron emitting radiolabeled tracer molecules and computed tomography to examine metabolic processes or to detect targets within the living body of a patient or experimental animal. Once injected, the tracer is monitored with a positron camera or a tomograph detector array. This technology can be more sensitive than scanning techniques such as magnetic resonance imaging (MRI), ultrasound imaging, or X-ray imaging. Some of the major clinical applications for PET are oncology, neurology, and cardiology.
Positron emitting compounds may be employed as markers and imaging agents because their presence and location are indicated by the annihilation of a nearby electron and the consequent emission of two oppositely oriented gamma rays. Gamma ray detectors can be used to detect the event and precisely determine its location.
Tracer molecules used in PET imaging are generally prepared by replacement of a group or atom in an unlabeled tracer with a radioisotope containing group or atom or by joining the tracer to a radioisotope containing atom (e.g. by chelation). Some common positron-emitting radioisotopes commonly used are: fluorine-18 (18F); carbon-11 (11C); nitrogen-13 (13N); and oxygen-15 (15O). In addition, 64Cu has been appended to tracer molecules using copper chelation chemistry (Chen et al. Bioconjug. Chem. (2004) 15: 41-49).
18F is a particularly desirable radioisotope for PET imaging since it has a longer half-life than 11C, 13N and 15O, readily forms covalent bonds, and has a short range beta+ emission that provides for high resolution in PET imaging.
Natural, stable fluorine is 19F. 18F has one less neutron for that number of protons, which is why it decays by positron emission.
18F is a fluorine radioisotope which is an important source of positrons. It has a mass of 18.0009380 u and its half-life is 109.771 minutes. It decays by positron emission 97% of the time and electron capture 3% of the time. Both modes of decay yield stable oxygen-18 (18O).
18F is an important isotope in the radiopharmaceutical industry. For example, it is synthesized into fluorodeoxyglucose (FDG) for use in positron emission tomography (PET scans). It is substituted for hydroxyl and used as a tracer in the scan. Its significance is due to both its short half-life and the emission of positrons when decaying.
In the radiopharmaceutical industry, the radioactive 18F must be made first as the fluoride anion (18F) in the cyclotron. This may be accomplished by bombardment of neo-20 with deuterons, but usually is done by proton bombardment of 18O-enriched water, with high energy protons (typically ˜18 MeV protons). This produces “carrier-free” dissolved 18F-fluoride (18F) ions in the water. Fluorine-18 is often substituted for a hydroxyl group in a radiotracer parent molecule. PET tracers often are or include a molecule of biological interest (a “biomolecule”). Biomolecules developed for use in PET have been numerous. They can be small molecules as ubiquitous as water, ammonia and glucose or more complex molecules intended for specific targeting in the patient, including labeled amino acids, nucleosides and receptor ligands. Specific examples include, but not limited to, 18F labeled fluorodeoxyglucose, methionine, deoxythymidine, L-DOPA, raclopride and Flumazenil. (Fowler J. S. and Wolf A. P. (1982), and The synthesis of carbon-11, fluorine-18 and nitrogen-13 labeled radiotracers for biomedical applications. Nucl. Sci. Ser. Natl Acad. Sci. Natl Res. Council Monogr. 1982).
The 109.8 minute half-life of 18F makes rapid and automated chemistry necessary after this point. 18F-fluoride anion (18F) is often converted to a form suitable as an agent in aliphatic nucleophilic displacements or aromatic substitution reactions. 18F may be combined with a metal ion complexing agent such as cryptand or a tetrabutyl ammonium salt, a triflate, or a positively charged counter ion (including Fr, H+, K+, Na+, etc).
Fluorination agents may be used in an appropriate solvent or cosolvent, including without limitation water, methanol, ethanol, THF, dimethylformamide (DMF), formamide, dimethylacetamide (DMSO), DMA, dioxane, acetonitrile, and pyridine.
In nucleophilic radiofluorination, the first major step is drying the aqueous [18F] fluoride which is commonly performed in the presence of a phase-transfer catalyst under azeotropic evaporation conditions (Coenen et al., J. Labelled Compd. Radiopharm., 1986, vol. 23, pgs. 455-467). The [18F] fluoride that is solubilized or dissolved in the target water is often adsorbed on an anion exchange resin and eluted, for example, with a potassium carbonate solution (Schlyer et al., Appl. Radiat. Isot., 1990, vol. 40, pgs. 1-6). One cryptatnd that is available commercially is 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8,8,8] hexacosan, with the tradename Kryptofix 222. Cryptand is a cage-like agent that has three ether ribs joining the nitrogens at each end. Alkali metals can be held very strongly inside the cage. The treatment with 18F is suitably effected in the presence of a suitable organic solvent such as acetonitrile, dimethylformamide, dimethylsulphoxide, tetrahydrofuran, dioxan, 1,2 dimethoxyethane, sulpholane, N-methylpyrrolidinineone,
In nucleophilic fluorination reactions, anhydrous conditions are required to avoid the competing reaction with hydroxide. [Aigbirhio et al 1995 J. Fluor. Chem. 70 pp 279-87]. The removal of water from the fluoride ion is referred to as making “naked” fluoride ion. This is regarded in the prior art relating to nucleophilic fluoridation as a step necessary to increase the reactivity of fluoride as well as to avoid hydroxylated by-products resulting from the presence of water [Moughamir et al 1998 Tett. Letts. 39 pp 7305-6; and Handbook of Radiopharmaceuticals 2003 Welch & Redvanly eds. ch. 6 pp 195-227). The removal of water from the [18F] Fluoride is referred to as making “naked” [18F] Fluoride. This is regarded in the prior art relating to nucleophilic fluoridation as a step necessary to increase the reactivity of fluoride as well as to avoid hydroxylated by-products resulting from the presence of water (Moughamir et al 1998 Tett Letts; 39: 7305-6).
The use of the cryptand to sequester the potassium ions avoids ion-pairing between free potassium and fluoride ions, making the fluoride anion more reactive. For example, [(2.2.2-cryptand) K+] 18F− is reacted with a protected mannose triflate; the fluoride anion displaces the triflate leaving group in an SN2 reaction, giving the protected fluorinated deoxyglucose. Base hydrolysis removes the acetyl protecting groups, giving the desired product 18FDG after removing the cryptand via ion-exchange (Fowler J S, Ido T (2002). “Initial and subsequent approach for the synthesis of 18FDG”. Semin Nucl Med 32 (1): 6-12; and Yu, S (2006). “Review of 18F-FDG synthesis and quality control”. Biomedical Imaging and Intervention Journal 2). To improve the reactivity of fluoride ion for fluoridation reactions a cationic counterion is added prior to the removal of water. The counterion should possess sufficient solubility within the anhydrous reaction solvent to maintain the solubility of the fluoride ion. Therefore, counterions that have been used include large but soft metal ions such as rubidium or caesium, potassium complexed with a cryptand such as Kryptofix™, or tetraalkylammonium salts. A preferred counterion for fluoridation reactions is potassium complexed with a cryptand such as Kryptofix™, because of its good solubility in anhydrous solvents and enhanced fluoride reactivity.
Fluorodeoxyglucose (18F) or fludeoxyglucose (18F), commonly abbreviated 18F-FDG or FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). Chemically, it is 2-deoxy-2-(18F) fluoro-D-glucose, a glucose analog, with the positron-emitting radioactive isotope fluorine-18 substituted for the normal hydroxyl group at the 2′ position in the glucose molecule. Synthesis of the FDG itself is not considered to be part of this invention and only a basic description of a process is included here.
Production of 18F-labeled FDG is, by now, well known. Information can be found in: 1) Fowler et al., “2-Deoxy-2-[18F]Fluoro-D-Glucose for Metabolic Studies: Current Status,” Appl. Radiat. Isotopes, vol. 37, no. 8, pp. 663-668 (1986); 2) Hamacher et al., “Efficient Stereospecific Synthesis of No-Carrier-Added 2-[18F]-Fluoro-2-Deoxy-D-Glucose Using Aminopolyether Supported Nucleophilic Substitution,” J. Nucl. Med., vol. 27, pp. 235-238 1986; 3) Coenen et al., “Recommendation for Practical Production of [2−18F]Fluoro-2-Deoxy-D-Glucose,” Appl. Radiat. Isotopes, vol. 38, no. 8, pp. 605-610 (1987) (a good review); 4) Knust et al., “Synthesis of 18F-2-deoxy-2-fluoro-D-glucose and 18F-3-deoxy-3-fluoro-D-glucose with no-carrier-added 18F-fluoride,” J. Radioanal. Nucl. Chem., vol. 132, no. 1, pp. 85-91 (1989); and 5) Hamacher et al., “Computer-aided Synthesis (CAS) of No-carrier-added 2-[18F]Fluoro-2-Deoxy-D-Glucose: An Efficient Automated System for the Aminopolyether-supported Nucleophilic Fluorination,” Appl. Radiat. Isotopes, vol. 41, no. 1, pp. 49-55 (1990). See also U.S. Pat. No. 6,567,492 to Kislelev al. (20 May 2003).
Several automatic processing systems capable of production of radiopharmaceuticals, such as 18F-labeled FDG, have also been described in: 1) U.S. Pat. No. 5,808,020 to Ferrieri et al. (15 Sep. 1998); 2) U.S. Pat. No. 6,599,484 to Zigler et al. (29 Jul. 2003); PCT pub. WO2004093652 by Buchanan et al. (2004 Nov. 4); and 3) German patent DE10320552 to Maeding et al. “Apparatus marking pharmaceutical substances with fluorine isotope, preparatory to positron-emission tomography, locates anion exchanger within measurement chamber” (2004 Nov. 25). Clinical Use of 18F-FDG
18F-FDG, as a glucose analog, is taken up by high-glucose-using cells such as brain, kidney, and cancer cells, where phosphorylation prevents the glucose from being released again from the cell, once it has been absorbed. The 2′ hydroxyl group (—OH) in normal glucose is needed for further glycolysis (metabolism of glucose by splitting it), but 18F-FDG is missing this 2′ hydroxyl. Thus, in common with its sister molecule 2-deoxy-D-glucose, FDG cannot be further metabolized in cells. The 18F-FDG-6-phosphate formed when 18F-FDG enters the cell thus cannot move out of the cell before radioactive decay. As a result, the distribution of 18F-FDG is a good reflection of the distribution of glucose uptake and phosphorylation by cells in the body. After 18F-FDG decays radioactively, however, its 2′-fluorine is converted to 18O−, and after picking up a proton H+ from a hydronium ion in its aqueous environment, the molecule becomes glucose-6-phosphate labeled with harmless nonradioactive “heavy oxygen” in the hydroxyl at the 2′ position. The new presence of a 2′ hydroxyl now allows it to be metabolized normally in the same way as ordinary glucose, producing non-radioactive end-products.
After 18F-FDG is injected into a patient, a PET scanner can form images of the distribution of FDG around the body. The images can be assessed by a nuclear medicine physician or radiologist to provide diagnoses of various medical conditions.
In PET imaging, 18F-FDG can be used for the assessment of glucose metabolism in the heart, lungs, and the brain. It is also used for imaging tumors in oncology, where a static 18F-FDG PET scan is performed and the tumor 18F-FDG uptake is analyzed in terms of Standardized Uptake Value (SUV). 18F-FDG is taken up by cells, phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly growing malignant tumours), and retained by tissues with high metabolic activity, such as most types of malignant tumours. As a result FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers, particularly in Hodgkin's disease, colorectal cancer, breast cancer, melanoma, lung cancer, and Alzheimer's disease.
Cryptands
Cryptands and other macrocyclic compounds such as crown ethers, spherands, cryptahemispherands, cavitands, calixarenes, resorcinorenes, cyclodextrines, porphyrines and others are well known. (Comprehensive Supramolecular Chemistry Vol. 1-10, Jean-Marie Lehn-1-Chairman of the Editorial Board, 1996 Elsevier Science Ltd.) Many of them are capable of forming stable complexes with ionic organic and inorganic molecules. Those properties make macrocyclic compounds candidates for various fields, for instance, catalysis, separations, sensors development and others. Cryptands (bicyclic macrocycles) have extremely high affinity to metal ions. The cryptand metal ion complexes are more stable than those formed by monocyclic ligands such as crown ethers (Izatt, R. M., et al., Chemical Reviews 91:1721-2085 (1991)). This high affinity of the cryptands to alkaline and alkaline earth metal ions in water makes them superior complexing agents for the processes where strong, fast and reversible metal ion binding is required. Examples of these processes include separation, preconcentration and detection of metal ions, analysis of radioactive isotopes, ion-exchange chromatography, phase-transfer catalysis, activation of anionic species and others.
Many strategies for the synthesis of macrocyclic compounds have been developed over the years (Comprehensive Supramolecular Chemistry Vol. 1-10, Jean-Marie Lehn—Chairman of the Editorial Board, 1996 Elsevier Science Ltd.; Krakowiak, K. E., et al., Israel Journal of Chemistry 32:3-13 (1992); Bradshaw, J S., et al., “Aza-Crown Macrocycles,” The Chemistry of Heterocyclic Compounds, Vol. 51, ed. Taylor, E. C., Wiley, New York, 1993; Haoyun, A., et al., Chemical Reviews 92:543-572 (1992)).
The Cryptands may be synthesised as described in US20040267009 A1, Bernard Dietrich, Jean-Marie Lehn, Jean Guilhem and Claudine Pascard, Tetrehedron Letters, 1989, Vol. 30, No. 31, pp 4125-4128, Paul H. Smith et al, J. Org. Chem., 1993, 58, 7939-7941, Jonathan W. Steed et al, 2004, Journal of the American Chemical Society, 126, 12395-12402, Bing-guang Zhang et al, Chem. Comm., 2004, 2206-2207.
Cryptands are cavity containing macromolecules which form stable complexes with alkali metal ions. For a given cation, the stability constant is largest for the cation which fits best into the cavity of the ligand. Thus stability maxima are found for Li[2.1.1]+, Na[2.2.1]+, and K[2.2.2]+ (Cox, B. G. Effects of substituents on the stability and kinetics of alkali metal cryptates in methanol. Inorganica Chimica Acta, 1981, 49, 153-158).